1. Field of the Invention
The present invention broadly relates to apparatus for exciting the atoms of a sample gas to higher atomic energy levels by transferring energy to the sample from a metastable gas. Such apparatus commonly is used for metastable transfer emission spectroscopy. More particularly, the present invention is directed to apparatus for mixing the sample gas with the metastable gas.
2. Description of the Prior Art
Metastable transfer emission spectroscopy (MTES) is a recently developed method of analyzing the elemental composition of a solid, liquid or gaseous sample. The method is described in U.S. Pat. No. 4,148,612 to Taylor et al.; U.S. Pat. No. 4,150,951 to Capelle et al.; and Aerospace Report No. ATR-78(8227)-1 by Capelle and Sutton entitled "Metastable Transfer Emission Spectroscopy: Method and Instrument for Detection and Measurement of Trace Material in Gas Flows," dated Mar. 30, 1978.
MTES involves the use of metastable gas, which may be defined as a gas having a substantial number of its atoms or molecules excited to atomic or molecular energy levels above the ground state, wherein the atoms or molecules remain in their excited states for a relatively long time, generally for a time ranging from a microsecond to a few seconds. Typically, the metastable gas is created by using a electromagnetic field to directly or indirectly excite a stable gas such as nitrogen or one of the noble gases.
In MTES, if the sample to be analyzed is not in gaseous form, it must be vaporized by some means. Various sample vaporization means are known in the art. In some cases the sample gas is entrained in an inert carrier gas, but such a combination will simply be referred to herein as the sample gas.
The sample gas and the metastable gas are directed to flow into a common mixing region. Upon mixture with the metastable gas, the atoms or molecules of the sample gas become excited to energy levels above the ground state via energy transfer from molecular collisions with excited metastable gas molecules. After being excited, the sample atoms or molecules almost immediately give up their excess atomic energy by emitting a photon of light and returning to the unexcited ground state. A spectrometer placed outside a transparent window near the mixing region analyzes the light emitted by the excited sample atoms. The wavelength and intensity of the emitted light respectively identify and quantify the constituents of the sample.
A critical component of any MTES system is the mixing means comprising the mixing region and the associated apparatus for mixing the sample gas with the metastable gas. One important requirement of the mixing means is that it must mix the two materials thoroughly enough so that the sample gas is exposed to and excited by the metastable gas with consistent efficiency. The Capelle patent discloses that this is so critical that the prior investigators' failure to develop the MTES technique may have been due to their ineffective means for dispensing the sample into the metastable gas flow.
Another important requirement of the mixing means is that it must minimize the amount of sample that is deposited on the walls of the apparatus instead of being mixed with the metastable gas. To the extent such deposition occurs, the number of excited sample atoms or molecules will be reduced, and hence the accuracy and sensitivity (i.e., the minimum detectable sample quantity) will be degraded. This requirement is difficult to satisfy because the sample atoms or molecules tend to adhere to any surfaces they contact.
The Capelle patent and the Capelle and Sutton report both disclose a mixing means which may be described as follows. The sample is vaporized in a furnace and swept up a vertical quartz glass tube by a carrier gas. A donut-shaped hollow ring, also made of quartz glass, is coaxially mounted within the tube above the furnace. The metastable gas is pumped through a length of ducting into the donut-shaped interior of the ring. From there, the metastable gas is injected into the tube through eight holes circumferentially spaced along the upward (i.e., downstream) surface of the donut and pointing slightly inward.
The Capelle mixing means has at least two disadvantages arising from the placement of the donut-shaped ring within the interior of the quartz tube. One disadvantage is that the ring partially obstructs the flow of sample gas through the quartz tube so that some of the sample may be deposited on the surface of the ring instead of mixing with the metastable gas. As discussed earlier, this would reduce the sensitivity of the instrument to small sample quantities. Another disadvantage is the impracticality of constructing a hollow ring within a quartz tube much smaller than the nine centimeter diameter tube described by Capelle. It is generally desirable to use smaller tubes to minimize the bulk of the apparatus.
Another disadvantage of the Capelle mixing means is that constructing the ring and mounting it within the tube are intricate, time-consuming, and expensive. Furthermore, the finished apparatus is very delicate and easily damaged.
A substantially different mixing means is disclosed in Duthler and Broida, "Excitation of Group Ia and IIb metal atoms by a Lewis Rayleigh nitrogen afterglow," Journal of Chemical Physics, Vol. 59, No. 1, pp. 167-174 (1973). The sample is vaporized in a furnace and swept upwards through the bottom of a stainless steel mixing chamber by a carrier gas. A Pyrex glass tube oriented perpendicular to the direction of sample gas flow extends from a point near the center of the mixing chamber to a source of metastable gas outside the chamber. The metastable gas is pumped through the Pyrex tube and enters the mixing chamber through a small orifice in the tube.
Duthler and Broida did not describe their apparaus a being useful for quantitative analysis of a sample. Instead, they disclosed using the apparatus to investigate the physics underlying the excitation of metal atoms by observing the relative intensities and pressure dependence of spectral lines. Since they were concerned only with measuring relative intensities instead of absolute intensities, their apparatus includes features that make it unsuitable for quantitative analysis by MTES. For example, the Pyrex tube through which the metastable gas is injected into the sample gas obstructs the sample flow so that a substantial fraction of the sample will be deposited thereon. In addition, their mixing chamber is composed of stainless steel, which will deactivate much of the metastable gas.
The Taylor patent does not describe any particular structure for the mixing means. It is depicted in the drawings only in generalized schematic form.